Silver halide emulsion containing a dihydroaromatic quaternary salt nucleating agent and the use thereof

ABSTRACT

Reactive cyclammonium quaternary salts including a dihydroaromatic ring nucleus, a dihydropyridinium nucleus for example, are useful nucleating agents in direct positive photographic emulsions. The salts include such compounds as a 1,2-dihydro-3-methyl-4-phenylprido( 2,1-b)-5-phenylbenzoxazolium salt. They are conveniently prepared by reacting an appropriate cyclammonium precursor compound in an organic solvent medium for a time sufficient to form the dihydroaromatic derivative.

United States Patent Kurtz et al.

SILVER HALIDE EMULSION CONTAINING A DIHYDROAROMATIC QUATERNARY SALT NUCLEATING AGENT AND THE USE THEREOF Inventors: Donald W. Kurtz; Donald W. Heseltine, both of Rochester, NY.

Assignee: Eastman Kodak Company, R0-

chester, N.Y.

Filed: Oct. 30, 1970 App1.No.: 85,706

US. Cl. ..96/107, 96/64, 260/239 Int. Cl. ..G03c 1/28 Field of Search ..96/64, 107, 94

References Cited l-laist et a1. ..96/64 3,367,778 2/1968 Berriman ..96/107 Primary Examiner-J. Travis Brown Assistant Examiner-Won 1-1. Louie, Jr.

Attorney-W. H. J. Kline, J. 11. Frederick and Dennis M. De Leo [57] ABSTRACT Reactive cyclammonium quaternary salts including a dihydroaromatic ring nucleus, a dihydropyridinium nucleus for example, are useful nucleating agents in direct positive photographic emulsions. The salts include such compounds as a 1,2-dihydro-3-methyl-4- phenylprido[2,l-b]-5-phenylbenzoxazolium salt. They are conveniently prepared by reacting an appropriate cyclammonium precursor compound in an organic solvent medium for a time sufficient to form the dihydroaromatic derivative.

8 Claims, No Drawings SILVER HALIDE EMULSION CONTAINING A DIHYDROAROMATIC QUATERNARY SALT NUCLEATING AGENT AND THE USE THEREOF The present invention relates to photography and more particularly to novel nucleating agents and directpositive photographic emulsions.

Certain photographic silver halide emulsions are used to prodce positive photographic images without requiring an intermediate processing step or a photographic negative.

Generally, these emulsions are termed direct-positive or reversal emulsions and, on exposure, they tend to form latent images predominantly inside the silver halide grains. The production of direct-positive photographic images is secured by selectively fogging the unexposed regions of a direct-positive photographic layer and developing a conventional photographic silver image corresponding to the unexposed, fogged regions. The fogging agent can be included in a fogging bath with which the imagewise exposed reversal emulsion is treated prior to image development. Alternatively, the fogging agent can be incorporated directly into the developer solution or it can be included in the reversal emulsion prior to coating a photographic layer.

Heretofore, fogging agents have generally been hydrazine compounds, such as those described in US. Pat. No. 2,563,785 (especially at column 1, lines 18-34 and column 3, line 28 to column 5, line 23), as constituents of a surface type silver halide developer, or those described in US. Pat. No. 2,588,982 (especially at column 1, lines 21-34 and at column 4, line 1 to column 5, line 52) as included in either a surface-type silver halide developer or in the-reversal emulsion itself.

When employing these techniques, and especially when the fogging agent is included in the reversal emulsion, there are certain disadvantages associated with presently known hydrazine fogging agents, or nucleating agents as they are interchangeably designated. To achieve a desirable degree of nucleation, relatively high nucleating agent concentrations, i.e., approximately 2 gm. per mole of silver as disclosed in US. Pat. No. 2,588,982, have been required. Such concentration levels promote non-uniform sensitometric characteristics, since undesirably large amounts of nucleating agent migrate into the developer solution. Additionally, once present in the developer, the nucleating agent tends to promote discoloration in non-image background areas.

It is thought that the described high nucleating agent concentrations are made necessary by the failure of nucleating compounds, i.e., hydrazines or the like, to be tightly adsorbed to the silver halide grain. The migration of nucleating agent into the developer solution, in addition to being promoted by heavy concentration is also stimulated by the high pH, conventionally in excess of 12 and ranging to 13 and above, which is required to maintain nucleation at an acceptable rate. Other more active reducing agents have been investigated as potential nucleating agents but these, produce overall fogging even at low p alues and are not operable as a selective nucleating agent in direct positive emulsions. It would be desirable to provide nucleating agents that fog, in image areas only, at pH ranges compatible with conventional emulsion addenda and at which the nucleating agent is substantially non-migrating.

Accordingly, in view of the above and related difficulties, it is an object of this invention to provide novel nucleating agents for photographic reversal emulsions.

It is another object of this invention to provide new, heterocyclic, dihydroaromatic quaternary salt nucleating agents for photographic silver halide reversal emulsions.

Still another object of the present invention is to provide novel nucleating agents which are useful in photographic reversal emulsions at low concentrations.

Yet an additional object of this invention is to provide novel heterocyclic, dihydroaromatic quaternary salt nucleating agents which are useful in direct-positive photographic elements and which do not produce substantial staining of non-image background regions.

Another object of this invention is to provide new quaternary salt nucleating agents useful at lower pH values.

Still another object of this invention is to provide new direct-positive silver halide emulsions wherein latent images are formed predominantly inside the silver halide grains.

These and other objects of the present invention will become increasingly apparent from a reading of the following specification and appended claims.

The objects of this invention are accomplished with direct-positive silver halide emulsions that form latent images predominantly inside the silver halide grains and which contain a dihydroaromatic quaternary salt nucleating agent comprising a l,2-dihydroaromatic heterocyclic ring nucleus including a quaternary nitrogen atom. Particularly advantageous 1,2- dihydroaromatic nuclei include such nuclei as a 1,2- dihydropyridinium nucleus. Especially preferred dihydroaromatic quaternary salt nucleating agents include those having the formula:

wherein:

a. Z represents the non-metallicatoms necessary to complete a heterocyclic nucleus containing a heterocyclic ring of from five to six atoms including the quaternary nitrogen atom, with the additional atoms of said heterocyclic ring being selected from either carbon, nitrogen, oxygen, sulfur or selenium,

' b. n represents a positive integer having a value of from 1 to 2,

0. when n is l, R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical, an alkoxy radical, an aryl radical, an aryloxy radical, and a carbamido radical and,

when n is 2, R represents an alkylene radical having from one to four carbon atoms,

d. each of R and R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical, and

e. X 6 represents an anion.

Exemplary heterocyclic nuclei of the type completed by the non-metallic atoms represented by Z and containing a heterocyclic ring of from five to six atoms including a quaternary nitrogen atom include such nuclei as, for example, an indole nucleus, an azole nucleus like an imidazole nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus, or a quinoline nucleus. Representative nuclei of these types include indolenine, members of the imidazole series such as benzimidazole compounds line -chlorobenzimidazole and also including compounds of the naphthimidazole series; those of the thiazole series such as thiazole, 4- methylthiazole, 4-phenylthiazole, S-methylthiazole, 5- phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, etc.; those of the benzothiazole series like benzothiazole, 4- chlorobenzothiazole, S-chlorobenzothiazole, 6- chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, 6-methylbenzothiazole, S-bromobenzothiazole, 6- bromobenzothiazole, 4-phenylbenzothiazole, S-phenylbenzothiazole, 4-methoxybenzothiazole, S-methoxybenzothiazole 6-m ethoxybenzothiazole, 5- iodobenzothiazole, 6-iodobenzothiazole, 4-ethoxybe nzothiazole 5 -ethoxybenzothiazole,

tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, S-hydroxybenzothiazole, 6-hydroxybenzothiazole, etc.; those of the naphthothiazole series such as naphtho [2,3-d]-thiazole, naphtho[2,l-d]thiazole, 8-methoxy 2, l -d naphthothiazole, etc.; those of the thionaphtheno-7',6',4,5-thiazole series like 4-methoxythionaphtheno-7 ',6',4,5-thiazole, etc., those of the oxazole series such as 4-methyloxazole, S-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, S-phenyloxazole, etc.; those of the benzoxazole series like benzoxazole, 5- chlorobenzoxazole, S-methylbenzoxazole, S-phenylbenzoxazole, -methylbenzoxazole, 5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, S-methoxybenzoxazole, S-ethoxybenzoxazole, 5-chlorobenzoxazole, 6-methoxybenzoxazole, S-hydroxybenzoxazole -hydroxybenzoxazole, etc.; those of the naphthoxazole series such as a-naphthoxazole; those of the selenazole series like 4-methylselenazole, 4-phenylselenazole, etc.; those of the benzoselenazole series like benzoselenazole, ,S-chlorobenzoselenazole, S-methoxybenzoselenazole, 5 -hydroxybenzoselenazole, tetrahydrobenzoselenazole, etc.; those of the naphthoselenazole series such as a-naphthoselenazole; and those of the quinoline series such as quinoline, etc.

The heterocyclic nucleus completed by the atoms represented by Z also include those nuclei containing a heterocyclic ring of from five to six atoms including a nitrogen atom and from four to five additional atoms of which from three to four atoms are carbon atoms and wherein the remaining additional atom is selected from either carbon, nitrogen, oxygen, sulfur or selenium atoms.

As defined herein, the term alkyl radical refers to alkyl radicals having from one to 18 carbon atoms like methyl, ethyl, propyl, butyl, hexyl, octyl, dodecyl or octadecyl, and preferably having from one to six carbon atoms. Included within the'defined alkyl radicals are substituted alkyl radicals like aralkyl radicals, e.g., benzyl or phenethyl and aryloxyalkyl radicals such as phenoxymethyl, etc. Alkylene radicals useful herein include those having from one to about four carbon atoms, i.e., methylene, ethylene, trimethylene, tetramethylene and the like. Also as defined herein the terms aryl radical and aryloxy radical refer to such radicals having from six to 10 carbon atoms in the aryl moiety, and preferably phenyl and phenoxy.

Alkoxy radicals, as defined herein, include alkoxy radicals having the above-mentioned alkyl radicals in the alkyl moiety and preferably from one to about-six carbon atoms, such as methoxy, ethoxy, propoxy, butoxy, pentoxy, isopropoxy, hexoxy and the like, including substituted alkoxy radicals like 2-chloroethoxy, 3- phenylpropoxy, carbalkoxy radicals such as carbomethoxy, carbethoxy, and the like.

The carbamido radicals recited herein include substituted carbamido radicals preferably having as substituents thereon, one or more alkyl and aryl radicals, or combinations thereof, as previously described.

The anions represented by X include a wide variety of acid anions including halide anions like bromide, chloride, and iodide, as well as additional anions, e.g., sulfates including sulfate, hydrosulfate, and lower alkyl sulfates like methylsulfate and ethylsulfate, aromatic sulfonates such as p-toluenesulfonate and benzenesulfonate, acid anions derived from carboxylic acids like acetate, trifluoroacetate, propionate, benzoate and a wide variety of other anions including anions such as, for example, perchlorate, cyanate, thiocyanate, sulfamate, etc. Especially advantageous anions are halide anions like those recited above.

Particularly useful nucleating agents of the type described herein include quaternary salts such as:

a. a l,2-dihydro-3-methyl-4-phenylpyrido[2, l -b] benzothiazolium salt,

b. a l,2-dihydro-3-methyl-4-phenylpyrido[2,1-b]-5- phenylbenzoxazolium salt,

c. a 1 ,2-dihydro-3 ,4-dimethylpyrido[2 ,1- b]benzothiazolium salt,

d. a 1,2-dihydro-3-methyl-4-phenoxypyrido[2,1-b] benzothiazolium salt,

e. a 4,4-ethylene bis(l,2-dihydro-3-methylpyrido[ 2, l -b]benzothiazolium salt) f. a l ,2-dihydro-3-methyl-4-phenylpyridoI 2, 1 -b] benzoselenazolium salt,

g. a 1 ,2-dihydro-3,4-diphenylpyrido[2, l -b]benzoxazolium salt,

h. a l,2-dihydro-3-phenyl-4-propylpyridonaphtho- [2,3-d]thiazolium salt,

i. a 1,2-dihydro-2,3-dimethyl-4-phenylpyrido[2,1-b] benzothiazolium salt,

j. a l,2-dihydro-2benzylbenzoxazolium 3-methyl-4- phenylpyrido- 2, l -b 1-5 -carbethoxybenzothiazolium salt,

k. a 1,2-dihydro-2-butyl-3-methyl-4-phenylpyrido- [2, l -b1-5-carbethoxybenzothiazolium salt,

1. 4,4-trimethylenebis( 1 ,2-dihydro-3-methylpyrido[2,l-b]benzothiazolium salt),

m. a 4,4'-tetramethylenebis(1,2-dihydro-2-ethyl-3- methylpyrido[2,1-b]benzothiazolium salt),

n. a l,2-dihydro-3-methyl-4-phenylpyrido[2,1-b]-5- (N-methyl-N-phenylcarbamido)benzothiazolium salt, and

o. a 1,2-dihydro-3,4-dimethylpyrido[2,l-b]-5-(N- ethyl-N-octadecylcarbamido)benzothiazolium salt.

I ll CHzCHC-R wherein each of Z, R, R, R and X e are as described herein, by reacting the quaternary salt to effect ring closure, i.e., for a time sufficient to form the 1,2- dihydroaromatic derivatives of the types mentioned elsewhere herein. The reaction is desirably carried out in a solvent medium using an inert organic solvent that exhibits moderate polarity and dissolves the reagent but which is not a solvent for the dihydroaromatic quaternary salt reaction product. Advantageous solvents include organic solvents having moderate polarity such as dimethylsulfoxide, acetonitrile, pyridine, dimethylacetamide and the like. No reaction catalysts are generally required and the reaction temperature is conveniently variable. Temperatures in excess of about 40 C. are desirable to promote the rate of reaction, and reflux temperatures are used to advantage, the only restriction being that reagents remain in solution and are not subjected to heating in excess of their decomposition temperatures. The reaction products generally precipitate from solution spontaneously, or are chemically precipitated such as by addition of ether, and can then be purified by such conventional means as solvent washing or sequential crystallization.

The useful 1,2-dihydr'oaromatic, including 1,2- dihydropyridinium quaternary salts are extremely advantageous as incorporated nucleating agents or fogging agents in silver halide reversal or direct positive emulsions. Such emulsions are utilized in photographic elements designed for reversal processing to prepare direct positive photographic images without an inter mediate developed negative image. Typical such direct positive emulsions are those that form latent images predominantly inside the silver halide grains.

The photographic emulsions generally used in reversal processes are gelatino-silver halide emulsions such as silver bromide, silver bromoiodide, silver chlorobromide or silver chlorobromoiodide emulsions. They need not contain sensitizing dyes although certain of such dyes can be added to reversal emulsions to widen spectral sensitization. Advantageous sensitizing dyes include those described in US. Pat. No. 2,497,876, especially between column 2, line 15, and column 4, line 20. Internal latent image-forming emulsions are typically undigested or if digested, the digestion is carried out without the use of surface chemical sensitizers. An emulsion of this type, known as Burtons emulsion is described in Wall Photographic Emulsions, 1927, pp. 5 2 and 53. A further type of suitable reversal emulsion is an internal latent image emulsion described in Davey and Knott, US. Pat. No. 2,592,250.

An internal latent image-forming emulsion can be prepared by first forming in one or more stages silver salt grainsconsisting at least partly of a silver salt which is more soluble in water than silver bromide, subsequently converting at least part of the grainsto silver bromide or silver bromoiodide, ripening, preferably in the absence of ammonia, and then either washing out some of the soluble salts or washing out the whole of the soluble salts, followed by the addition of soluble salts such as soluble chloride, bromide or iodide to prepare a composite silver chlorobromide, bromoiodide or chlorobromoiodide reversal emulsion. Suitable silver chlorobromide internal image emulsions also can be prepared by simultaneously adding an aqueous solution of silver nitrate and an aqueoussolution containing potassium chloride and potassium bromide to an agitated aqueous gelatin solution as described in Example 4 of Luckey et al., US. Pat. No. 2,996,382.

An internal latent image type of silver halide emulsion can be defined as one which, when a test portion is exposed to a light intensity scale for a fixed time up to about 1 second and developed for 4 minutes at 20 C in an ordinary surface" developer, one which develops an image only on the surface of the grains of an internal latent image emulsion, which developer type is. exemplified by one having the formula:

p-hydroxyphenylglycine 10 grams sodium carbonate (crystals) 100 grams water to 1 liter exhibits a maximum density not greater than one-fifth the maximum density obtained when the same emulsion is equally exposed and developed for 3 minutes at 20 C in an internal type of developer, one which develops an image inside the grains of an internal latent image emulsion, which developer type is exemplified by one having the formula:

hydroquinone l5 grams monomethyl-p-aminophenol sulfate 15 grams sodium sulfite (anhyd) 50 grams potassium bromide 10 grams sodium hydroxide 25 grams sodium thiosulfate (crystals) 20 grams water to l liter Preferably the maximum density obtained with the surface" developer is not greater than onetenth the maximum density obtained when the same emulsion is developed in the internal type developer. Stated conversely, an internal latent image" emulsion, when developed in an internal type developer exhibits a maximum density at least 5 and preferably at least 10 times the maximum density obtained when the emulsion is exposed in the same way and developed in a surface developer.

The subject quaternary salts that operate advantageously as nucleating agents or fogging agents in internal latent image forming (reversal) emulsions are conveniently added to a silver halide reversalemulsion, such as, for example, thosedescribed in US. Pat. Nos. 2,566,l80 or 2,592,250 in amounts designed to produce adequate fog in the form of minute surface of silver being preferred in typical situations. These 1 concentrations are according to conventional practice, however and with particular reversal emulsions or processing conditions more widely varying fogging agent concentrations, i.e., higher concentrations, may be desirable.

Silver halide emulsions into which the heterocyclic quaternary salt nucleating agents of this invention are advantageously incorporated typically contain other chemical addenda such as, for example, carrier vehi-.

cles, spectral sensitizing dyes, coating aids, brighteners, hardeners, developing agents and the like addenda. Desirable carrier vehicles include any of the hydrophilic, water-permeable binding materials suitable for this purpose. Suitable materials include gelating, colloidal albumin, polyvinyl compounds, cellulose derivatives, acrylamide polymers and the like, alone or in combination and mixture. The binding agents for the emulsion layer of the photographic element can also contain dispersed polymerized vinyl compounds. Certain of such compounds are disclosed, for example, in US. Pat. Nos. 3,142,568 of Nottorf issued July 28, 1964; 3,193,386 of White issued July 6, 1965; 3,062,672 of l-louch et al. issued Nov. 6, 1962; and- 3,220,844 of Houck et al. issued Nov. 30, 1965; and include the water-insoluble polymers and latex copolymers of alkyl I acrylates and methacrylates, acrylic acid sulfoalkyl acrylates or methacrylates and the like.

Spectral sensitizing dyesuseful in sensitizing the silver halide emulsions employed in this invention are described in, for example, US. Pat. Nos. 2,526,632 of Brooker and White issued Oct. 24, 1950 and 2,503,776 of Sprague issued Apr. 11, 1950. Spectral sensitizers which can be used are the cyanines, merocyanines, complex (trinuclearlcyanines, complex (trinuclear) merocyanines, styryls and hemicyanines.

The photographic silver halide emulsions or coatings disclosed herein can also contain non-ionic, anionic and/or amphoteric coating aids. Some useful coating aids include, for example, saponin, alkyl substituted aryloxyalkyleneethyl sulfonates of the type described in US. Pat. No. 2,600,831 issued June 17, 1952,

' maleopimarates of the type described in US. Pat. No.

2,823,123, issued Feb. 1 l 1958, taurine derivatives of the type described in US. Pat. No. 2,739,891, issued on Mar. 27, 1956, and alkyl aminopropionates of the type described in US. Pat. No. 3, 133, 816 issued May 19, 1964. Typical of still other good coating aids and surfactants which can be employed in the emulsions of this invention include the alkylphenoxy poly(hydroxyalkylene oxides) such as alkylphenoxy poly(glycidols) having from about 5 to about 12 glycidol units, for exmembered ring such as derivatives of stilbene, stilbenetriazole, triazine stilbene, coumarin, triazinylamino coumarin, oxazole, benzidene, benzimidazole, benzothiazole, benzoxazole,

pyrazoline, naphthalic acid imide, etc. Exemplary brighteners include such compounds as those described in US. Pat. No. 2,933,390 and in 11.5. Pat. No. 3,406,070.

As well as including the above-mentioned addenda, the silver halide emulsions employed in this invention can be hardened with any suitable hardener or combinations such as, e.g., formaldehyde, mucochloric acid, glutaraldehyde, maleic dialdehyde, aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, vinyl sulfones, oxypolysaccharides such as oxysta'rch, oxy plant gums, inorganic hardeners such as chromium salts, and the like. Developing agents of the types suitable for inclusionin a silver halide emulsion are described hereinbelow.

To prepare direct positive photographic elementsthat can be reversal processed, an internal latent image-forming silver halide emulsion such as described hereinabove, is coated onto a typical photographic support material and dried. Advantageous support materials include conventional photographic film base materials like cellulose esters such as cellulose acetate, cellulose 'triacetate, cellulose acetate butyrate, etc., poly-a-olefins like polyethylene and polycarbonates, polyesters such as poly(ethylene terephthalate) as well as metals such 'as zinc and aluminum and paper including polyethylene and polypropylene coated papers. Other support materials that aresuitably used herein are known in the art. 1

The nucleating agents of the invention can be used in reversal color as well as black-and-while silver halide Pat. Nos. 3,227,550 and 3,227,552. Spectral sensitizing dyes can be used to sensitize the silver halide in'the usual manner. In a multilayer element, the red recording emulsion layer can contain a coupler such as 5-(pamylphenoxy benzene sulfonamino)-l-naphthol,- the green-recording emulsion layer, a coupler like 2- cyanoacetyl-S-(p-sec. amylbenzoylamino) coumarone andthe blue-recording emulsion layer, a coupler such as N-amyl-p-benzoylacetamino benzene sulfonate. Additional couplers are well known in the art. If desired,

the color couplers can be incorporated into the developer solution, as described in Mannes et al., US. Pat. No. 2,252,718 and Glass et al., US. Pat. No. 2,507,154, such an arrangement necessitating multiple exposures and color developing steps.

After an imagewise exposure, the direct positive photographic elements containing the subject nucleating agents are immersed in a conventional surface developer solution having relatively low solvent action on silver halide whereupon a positive photographic image is formed. Such surface developers can advantageously contain image-enhancing compounds which increase maximum image density and lower I minimum image density such as the benzotriazoles described in Stauffer, U.S. Pat. No. 2,497,917.

lf color images are to be prepared, the developer solution typically contains a p-phenylenediamine color developing agent such as a 4-amino-N-dia1ky1aniline like those described in Mees, The Theory of the Photographic Process, 3rd Ed. (1966) pp. 294-295. With a color developer, one or more colored dye images are produced depending upon the construction of the particular photographic element. In polychromatic color elements, cyan, magenta and yellow dye images are typically produced in the red sensitive, green sensitive and blue sensitive layers, respectively. Remaining silver is first converted to a soluble silver salt and removed in the usual way by treatment with a fixing agent such as sodium thiosulfate.

The following examples are included for a further understanding of the invention.

Example 1 1 ,2-Dihydro-3-methyl-4-phenylpyrido[2,1-b ]benzothiazolium bromide is prepared by reacting 3- (2-acetylethyl)-2-benzy1benzothiazolium bromide (1.0 g., 0.0027 m) dissolved in dimethyl sulfoxide (25 ml.). The reaction mixture is heated to reflux for several minutes and cooled. The resulting tan solid precipitate is filtered and washed with ether and dried to give 0.7 g., 72 percent of product m.p. 208-210 C.

Example 2 1 ,2-Dihydro-3-methyl-4-pheylpyrido[2,1-b]-5-phenylbenzoxazolium bromide can be prepared by reacting 3-(2-acetylethyl)-2-benzyl-5-phenylbenzoxazolium bromide (5.0 g., 0.01 l m) dissolved in acetonitrile (75 ml.). The reaction mixture is stirred at about 30 C. for about 124 hours, after which ether is added to the solution and the resultant precipitate is filtered, washed and dried to give 4.27 g., 93 percent of product, m.p. 269 C.

Example 3 The following dihydroaromatic quaternary salts are prepared according to one of the above procedures as summarized in the following table:

EXAMPLE 4 Six dihydroaromatic quaternary salts,

a. l ,2-Dihydro-3-methy1-4-pheny1pyrido[2, 1 -b]. benzothiazolium bromide,

b. 1 ,2-Dihydro-3-methyl-4-phenylpyrido[2,1-b]5- phenylbenzoxazolium bromide,

c. 1,2-Dihydro-3,4-dimethylpyrido[2,1- b]benzothiazo1ium bromide,

d. 1 ,2-Dihydro-3-methyl-4-phenoxypyrido[2, l -b] benzothiazolium bromide,

e. 4,4'-Ethylenebis( l ,2-dihydro-3-methylpyrido[2,1- b]benzothiazolium bromide),

f. 1 ,2-Dihydro-3-methyl-4-phenylpyrido[2,.1-b] benzoselenazolium bromide, are tested for nucleation activity in a gelatino-silver chlorobromoiodide internal image reversal emulsion of the type described by Davy and Knott in Example 1 of U.S. Pat. No. 2,592,250. lThey are added from solution to portions of the emulsion in the range of from 200-400 mg/mole Ag as specified in the following table, and the emulsionsare coated on a cellulose acetate support material and dried. The resultant photographic elements, including control elements coated without nucleating agent, are exposed in an Eastman model aB sensitometer. Subsequent to exposing, certain of the elements are developed for about 1 minute at 38 C. in a developing solution having the formula:

p-methylaminophenol 2 g. sodium sulfite (dessicated) g'. hydroquinone 8 g. sodium carbonate (monohydrate) 52.5 g. potassium bromide 5 g.

water to make and the remaining elements are developed in a similar fashion, but with the developer pH adjusted to 12.1 with sodium hydroxide. The stock developer pH is 10.5. The developed images are stabilized by treatment for about 3 minutes at 20 C. with a fixing bath having the formula:

sodium thiosulfate 240 g. sodium sulfite (dessicated) 15 g. acetic acid (28%) 48 ml. boric acid crystals 7.5 g. potassium alum 15 g.

water to make 1 liter Sensitometric results are summarized in the following table:

TABLE I Action of the Nucleating Agents Compound solvent added Reversal Image compoundf 40f) acetonilrile 10.5 1.28 0.": ncelnnitrile l2.l 322 (HR The tabulated data demonstrates the desirable nucleation potential of these compounds at low concentrations. Additionally, acceptable reversal images can be obtained at lower pH values than those required previously. Contamination of the developer solution by fogging agent is not a problem.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A direct positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a dihydroaromatic quaternary salt having the formula:

wherein:

a. Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing a heterocyclic ring of from five to six atoms including the quaternary nitrogen atom, with the additional atoms of said heterocyclic ring being selected from the group consisting of carbon, nitrogen, oxygen, sulfur and selenium,

b. n represents a positive integer having a value of from 1 to 2,

c. when n is l, R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical, an alkoxy radical, an aryl radical, an aryloxy radical, and a carbamido radical, and,

when n is 2, R represents an alkylene radical having from one to four carbon atoms,

d. each of R and R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical, and

e. X represents an anion.

2. A, direct positive silver halide emulsion as described in claim 1, where X represents a halide anion.

3. A direct positive silver halide emulsion as described in claim 1 wherein the emulsion has a dyeforming coupler contiguous thereto.

4; A direct positive silver halide emulsion as described in claim 1 wherein the dihydroaromatic quaternary salt is present at a concentration of from about 50 mg to about 1,500 mg per mole of silver halide in said emulsion.

5. A direct positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a dihydroaromatic quaternary salt selected from the group consisting of:

l,2-Dihydro-3-methyl-4-phenylpyrido[ 2, l -b]- bcnmthiazolium bromide, b. l ,2-Dihydro-3-methyl-4-phenylpyridol2,1-b]-5- phenylbenzoxalz/olium bromide, c. l ,2-Dihydro-3 ,4-dimethylpyndo 2, l -b]- benzothiazolium bromide,

1 ,2-Dihydro-3-methyl-4-phenoxy-pyrido 2, l -b benzothiazolium bromide e. 4,4-Ethylenebis( l ,Z-dihydro-3-methylpyrido-[ 2,1

-b]benzothiazolium bromide),

f. l ,2-Dihydro-3-methyl-4-phenylpyrido 2 l -b 1 benzoselenazolium bromide.

6. A direct positive silver halide emulsion as described in claim 1 wherein the emulsion is a gelatino silver halide emulsion.

7. A direct-positive silver halide emulsion as described in claim 1 wherein the silver halide comprises silver chlorobromoiodide.

8. A photographic image forming process for the preparation. of direct positive images on an imagewise exposed photographic element comprising a support having coated thereon a light-sensitive layer comprising a light-sensitive photographic silver halide emulsion which forms latent images predominantly inside the silver halide grains, said emulsion comprising silver halide grains having a predominant amount of light sensitivity in the internal portion of said grains, and said emulsion containing a nucleating amount of a dihydroaromatic quaternary salt of the formula:

wherein:

a. Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing a heterocyclic ring of from five to .six atoms including the quaternary nitrogen atom,-with the additional atoms of said heterocyclic ring being selected from the group consisting of carbon, nitrogen, oxygen, sulfur and selenium,

b. n represents a positive integer having a value of from 1 to 2,

c. when n is l, R represents a member selected from the group consisting of a hydrogen atom, an alkyl radical, an alkoxy radical, an aryl radical, an aryloxy radical, and a carbamido radical, and, when n is 2, R represents an alkylene radical having from one to four carbon atoms,

. each of R and R represents a member selected from the group consisting of a hydrogen atom, an allgl radical and an aryl radical, and

e. X represents an anion, which process comprises contacting the imagewise exposed photographic element with a surface type photographic developer and thereby producing a visiblephotographic image in the unexposed areas of said element. 

1. A direct positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a dihydroaromatic quaternary salt having the formula:
 2. A direct positive silver halide emulsion as described in claim 1, where X represents a halide anion.
 3. A direct positive silver halide emulsion as described in claim 1 wherein the emulsion has a dye-forming coupler contiguous thereto.
 4. A direct positive silver halide emulsion as described in claim 1 wherein the dihydroaromatic quaternary salt is present at a concentration of from about 50 mg to about 1,500 mg per mole of silver halide in said emulsion.
 5. A direct positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a dihydroaromatic quaternary salt selected from the group consisting of: a. 1,2-Dihydro-3-methyl-4-phenylpyrido(2,1-b)-benzothiazolium bromide, b. 1,2-Dihydro-3-methyl-4-phenylpyrido(2,1-b)-5-phenylbenzoxalzolium bromide, c. 1,2-Dihydro-3,4-dimethylpyrido(2,1-b)-benzothiazolium bromide, d. 1,2-Dihydro-3-methyl-4-phenoxy-pyrido(2,1-b)-benzothiazolium bromide e. 4,4''-Ethylenebis(1,2-dihydro-3-methylpyrido-(2,1-b)benzothiazolium bromide), f. 1,2-Dihydro-3-methyl-4-phenylpyrido(2,1-b)-benzoselenazolium bromide.
 6. A direct positive silver halide emulsion as described in claim 1 wherein the emulsion is a gelatino silver halide emulsion.
 7. A direct-positive silver halide emulsion as described in claim 1 wherein the silver halide comprises silver chlorobromoiodide. 